Servo control amplifier



'Jan. 16, 1951 L. E. BARTON SERVO- CONTROL AMPLIFIER 2 Sheets-Sheet 1 Filed Jan. 15, 1946 INVENTOR L9H E. Barton BY 08M ATTORNEY Jan. 16, 1951 E. BARTON SERVO CONTROL AMPLIFIER 2 SheetLs-Sheet 2 Filed Jan. 15, 1946 C. A g a a #7 -Wi YEP fi u I g 3% 1 J) nmmw 2 EDI/73% 07PM!!! iya INVENTOR L9 lllilBar ton BY 0&1

ATTORNEY Lana Patented Jan. 16, 1951 UNITED STATES PATENT OFFICE SERVO CONTROL AIHPLIFIER Loy E. Barton, Collingswood, N. J., assignor to Radio Corporation of America, a corporation of Delaware Application January 15, 1946, Serial No. 841,332

Claims. 1

The subject invention relates to servo control amplifiers of the type used in servo mechanisms and which are capable of selectively producing output potentials of difierent polarity under the influence of a control voltage.

Servo mechanisms have commonly been used for the purpose of controlling the angular position of an antenna array, for example, from a remote point through electrical means. Such systems employ a self-balancing arrangement by means of which the instantaneous position of the antenna is electrically compared to that of a manual control device to produce a differential or error voltage which sets into motion a sequence of events which automatically corrects the difference in the relative position of the control device and the antenna. The error" voltage is generally of small amplitude and power capacity and is insufilcient to operate directly the reversible driving motor connected to the antenna. Consequently, it is necessary to provide a servo control amplifier capable of producing suflicient power in its output circuit to drive the motor in one direction or the other in accordance with the error voltage produced by the system. It is the primary purpose of this invention to provide an improved servo control amplifier of this type,

capable of producing an output current whose phase reverses in accordance with the control signal and is of suflicient amplitude to operate a reversible motor designed to rotate the large mass of an antenna array or to rotate by remote control other similar devices.

A further object of this invention is to provide an improved servo control amplifier in which alternating voltages may be applied directly to the amplifier tubes.

A still further object of this invention is to provide a servo control amplifier adapted for utilization with thermionic tubes as well as 'Ihyratrons.

A still further object of this invention is to provide a control amplifier which is readily adjustable so that an output current of the desired amplitude may be obtained.

The above objects, as well as other objects which will be seen from a further study of this invention, are achieved by applying alternating currents of like phase to the plate electrodesv of two output tubes while at the same time applying a biasing potential plus alternating voltages of opposite phase to the respective grid electrodes of the output tubes, one ,of the input voltages which are applied to the grid electrodes being reversible through 180 in accordance with the desired direction of movement of the load or an tenna. Conversely, the voltages of like phase may be applied to the grid electrodes, and the out of phase voltages to the plates.

This invention will be better understood from the following description when considered in connection with the accompanying drawings. Its scope is indicated by the appended claims.

Referring to the drawings,

Figure 1 is a circuit diagram of a servo control amplifier in accordance with this invention,

Figures 2 and 3 are voltage curves illustrating the operation of the device shown in Fig. 1.

Figure 4 is a circuit diagram of a modification of this invention,

Figure '5 is a series of voltage curves illustrating the operation of the modification shownin Fig. 4, and

Figure 6 is a circuit diagram of an alternate means of deriving output from the device shown in Fig. 4.

Referring now to Fig. 1, the circuit connections for an embodiment of this invention will be described. A pair of input terminals 6, 3 are coupled respectively to ground and the input electrode of a thermionic tube 5. The output electrode of tube 5 is connected to a source of positive potential, not shown, through the primary of a coupling transformer l. The secondary of this transformer is center tapped, the center tap being connected to ground through resistors 9, II. The outer ends of the secondary are connected, respectively, to the input electrodes of a. pair of control tubes l3, 15. The cathode electrodes of the latter tubes are connected to ground through by-passed biasing resistors in the customary manner. The output electrodesof tubes i3 and [5 are connected, respectively, to the ends of the primary winding of an output transformer ll, the center point I9 01' which is connected to one side of the high voltage secondary 2| of a power transformer 23, the other side being grounded. Condensers 25 and 21 are connected across the two halves of tlfe primary of transformer l1 and tune the circuits to resonance at the operating frequency of cycles per second. The secondary of transformer i1 is also tuned by a capacitor 29 and is connected to one field coil 3| of an induction motor 33. The other field coil 35 of the inductionmotor is connected to a source of 60 cycle current through the phase shifting condenser 30 as shown. A potentiometer 31 is connected across the high voltage secondary of transformer 23 and its contact arm is connected to the cathode of a diode rectifier tube 39, the

transformer I to produce a D.-C. bias voltage across resistors 9, II. This bias is applied to tubes 5, I3 and I as shown.

The operation of the servo control amplifier illustrated i Fig. 1 will be better understood by making reference at the same time to the curves illustrated in Figs. 2 and 3. The device requires for its operation an alternating input voltage whose phase is changed 180 to determine the desired direction of rotation of the output motor ll which drives the load 43 in the desired direction. The reversible nature of this input voltage is indicated in Fig. 1 by showing a reversing switch connected to the input, but it must be understood that when connected in a normal servo system the alternating error" voltage produced by the system will inherently have this phase reversing feature. For the control ampliher to serve its intended purpose it is only necessary to show that when an alternating voltage of a given phase is applied to the input terminals I, 3, motor 33 will rotate in a given direction and when the input voltage is of the opposite phase the motor 33 will rotate in the opposite direction.

Transformer 23 applies to the anode electrodes of the output tubes 13 and 15, through the respective portions 0! the secondary oi transformer ll, voltages which are in phase as indicated by the respective curves Ep in Figs. 2 and 3. An adjustable negative grid bias is applied to the grid electrodes I3 and I! by the action of the diode rectifier It. This voltage is indicated by the dotted "bias line shown in Figs. 2 and 3. The input voltage is applied in phase opposition by transformer i to the grid electrodes of the amplifier tubes I! and It as indicated by the curves E; in

Figs. 2 and 3 respectively. It will be observed that the grid and plate voltages of the tube l3, for example, are therefore in phase and the tube will become conductive whenever the applied grid voltage is of sufilcient amplitude to bring the eflective grid voltage to the value necessary to operate the particular tube being used. The periods during which tube It becomes conductive, on the assumption that conductivity occurs at zero grid bias, are indicated by the shaded areas in Fig. 2. This output current passes through one half the primary of transformer H in the direction indicated by the solid arrow and induces in the secondary a like current which fiows through the motor field winding 3| in the direction also indicated by the solid arrow. Tube ll remains non-conductive, however, since at all times the grid voltage is negative when the plate voltage is positive and vice versa, as shown in Fig. 3. The purpose oi the tuning capacitors 21 and 29 is to smooth out the current impulses and to produce in the field winding 3| a substantially sine wave current.

7 Motor 33 is a standard two-winding induction motor which rotates in one direction when the respective field windings are energized by currents of a given relative phase, and which rotates in the opposite direction when the phase of the current energizing either of the windings is reversed. Since winding 35 is connected to a constant source of 60 cycle current, the direction of rotation will be determined by the relative instantaneous current flow in winding II. The 90 phase relation for one phase of the motor is ob-- tained by the condenser 30 in series with motor winding 35.

It the phase of the applied input voltage is reversed 180, as is the case when the error voltage reverses, it will be understood that tube II becomes non-conductive and tube It now be comes conductive. As a result, no current will flow in the output circuit of tube II, but a current will fiow through half the primary of transformer I! in the direction indicated by the dotted arrow which will induce in the secondary winding a corresponding current also flowing in the direction shown by the dotted arrow which energizes the field winding II with a current of 0D- posite phase to that in the case previously dis cussed. As a result the direction of rotation of the motor 33 will be opposite to that previously produced.

By readjusting the bias voltage produced by rectifier 38 a different mode of operation is available which has certain advantages. In the absense of input voltage, potentiometer I1 is adjusted to reduce the negative grid bias to such a value that both tubes l3 and it become conductive during the half cycle the applied plate voltage is positive. The resultant rapidly reversing current tends to lock the motor, preventing its rotation. A braking effect is therefore produced which is useful in quickly stopping the system when the balance point has been reached. The applied input voltage shuts off one tube or the other, and thus, when an error voltage is received, the system works as described above.

An alternative embodiment of this invention is illustrated in Fig. 4 to which reference is now made. Input voltages from the servo system are applied to input terminals I and 3 as before. A double triode vacuum tube 45 of the 6N7 type is used for the input amplifier and rectifier combined, thus obviating the need of the additional rectifier tube 39 employed in the embodiment previousLv described. Input is applied to both grid electrodes in parallel, the output electrodes being connected to the outer terminals of the secondary of a high voltage transformer 41, the primary of which is connected to a standard source of 60 cycle current. The cathode electrodes of tube 45 are connected together and to ground through a biasing resistor 41. Two additional resistors 49 and BI are also connected in series between the cathodes of tube 45 and ground. Their mid-point is connected to input terminal 3 to provide bias for tube 45 through the input potentiometer 48. The cathodes are also connected through coupling resistors 53, 55 to the grid electrodes of a pair of Thyratrons I1 and 59.

These grid electrodes are also cross-connected to the outer terminals of transformer 41 through coupling resistors 6|, 63 respectively. The anode electrodes of the Thyratron tubes are connected. respectively, to the high potential terminals of the secondary of transformer 41 through field windings and 61 of an induction motor 69. The field windings are preferably tuned to resonance by capacitors II and 13 respectively.

The operation of the servo control amplifier illustrated in Fig. 4 will be understood in connection with the voltage curves shown in Fig. 5. The zero motor speed voltages applied to the Thyratrons 51 and 59 are illustrated in Figs. 5 (a) and (c), respectively. It will be seen that the plate voltages are in phase opposition and that the grid voltages which result from the potentials applied through resistors SI and 83. not considering the effect of tube 45, indicated by curves E3, are in phase opposition to the plate potentials applied to the respective tubes. Resistor 41 preferably has a low value as compared to resistors 55 and 63, or BI- and SI, and it has little or no effect on the value of the grid volt- 5 ages applied to the Thyratron tubes, not considering the action of tube 45. The latter tube, however, rectifies both halves oi'- the A. C. voltage applied to the plate electrodes and a positive pulse is produced during each half wave due to the rectifying action 01. the tube. These pulses are superimposed on the Thyratron grids and are as shown in Figs. 5 (a) and (c) and are indicated as steady state input pulses." It will be observed that so long as the amplitude of these pulses does not exceed the grid bias E, the grid bias will remain negative and no current will flow in the output.

The application oian alternating voltage to the input terminals, however, causes an additional input voltage to be applied which will increase the efiective Thyratron grid voltage during one hal! 01 each cycle, and decrease the effective grid voltage during'the other half of each cycle as shown in Fig. 5 (b) corresponding to the operation of tube 51 for a given'phase of applied input voltage. It will be observed"that the added positive impulse in the first hali cycle will cause the grid of tube 51 to become positive at the same time its plate is positive, and current will flow during this period. At the same time, however, tube 59 will remain non-conductive since, as shown in Fig. 5 during the first half-cycle its plate is negative and during the second half-cycle the control impulse of decreasedamplitude is insufficient to overcome the applied steady state grid bias ng voltage Eg. As a result, for the assumed phase of input voltage current will fiow through field winding 65 of motor 59, but field winding 61 will not be activated. If the phase of the input voltage is reversed, the conditions will likewise be reversed and the voltage applied to tube 59 will be as indicated in Fig. (d) The tube will become conductive during the second half cycle 01' the applied plate voltage and current will flow through winding 61 ofv motor 59. whereas winding 65 will not be activated. Motor 69 in this case is a Telechron type motor having two separate field windings, and direction of rotation is determined in accordance with which of the two windings is energized. Consequently when the amplifier is energized by a control voltage of a given phase, the motor will rotate in one direction, and when the amplifier is energized by a control voltage 01 an opposite phase, the motor will rotate in the opposite direction.

An alternative utilization of the circuit illustrated in Fig. 4 is shown in Fig. 6, to which reference is now made. The circuit to the left of the dotted vertical line in Figs. 4 and 6 is as shown in Fig. 4. If it is desired to employ a standard type induction motor similar to that described in connection with Fig. 1 above, the circuit may be modified as shown. It should be kept in mind that the type motor to be employed in this case requires both fields to be, energized, and the direction of rotation is determined by the relative phase of the currents in the two fields. This is accomplished by connecting the plate electrodes of the Thyratron tubes 51 and 59 directly to the high potential terminals of transformer 41, while the grid electrodes are biased through a cross-connection to opposite ends 01 the secondary through resistors 6| and 63 as before. The field winding 15 of induction motor 11 is connected between the center tap of the high potential secondary and the cathodes of the Thyratron tubes and may be tuned by a capacitor 19 as before. The second field winding 6 Si is energized directly from the cycle current by suitable connections across the primary of transformer 41 through a 90 phase shitting condenser 80.

Referring again to Figs. 5 (b) and 5 (d) it will be observed that when tube 51 is conductive, the current flows in the output circuit in a given direction during the first half of the alternating cycle and that when current flows in tube 59 the period of conductivity is initiated in the second half cycle, and current flows in the output circuit in the same direction during the second half of the alternating cycle. When considered in relation to the current flowing in field winding til, it will therefore be observed that the instantaneous direction of current in field winding 15 will go through a phase reversal depending upon which of the Thyratron tubes is conductive. As pointed out above, this in turn depends upon the phase of the applied alternating voltage. Consequently, for an applied voltage of a given phase, the motor 11 will rotate in one direction and for an applied voltage of the opposite phase, the motor 11 will rotate in the opposite direction.

The method of operation in which the output tubes are both conducting in the absence of an error voltage. as described above with reference to the embodiment illustrated in Fig. 1, may also be employed in the embodiments illustrated in Figs. 4 and 6. By suitably adiusting the fixed grid bias the tubes become operative during the zero error signal period, producing a de irable braking effect which quick y stops the motor.

I have hus described an improved s rvo control amplifier, together with several modifications thereof, in which a small control voltage produced. for example, in a servo system, is employed to control thermionic or Thyratron tubes so as to control the direction of rotation of a motor whi h, in accordance wi+h the usual custom,

is employed to rotate a device in such a direction as to return the servo system to a balanced condition. The control amplifiers are A. C. operated and do not reouire special transformers or other equipment in their operation.

I claim as my invention:

1. In a device of the character described, the combination which includes a pair of discharge devices having a cathode, anode and grid electrodes, a first source of alternating current potentials connected between the anode and cathode electrodes of said devices, a second source of alternating current potentials connected between the grid and cathode electrodes of said devices, said connections being such that in one of said devices ,the cathode-grid voltage is in phase with the cathode-plate voltage and in the other of said devices the corresponding voltages are in phase opposition whereby one or the other of said devices may become conductive depending on the phase of the potential applied by said second source, output means in circuit with said anode electrodes and responding in a first mode to current flowing in one of said devices and in a second mode to current flowing in the other of said devices, a rectifier connected to said first source of potentials for developing a rectified voltage, and means for applying said rectified voltage between said grid and cathode electrodes of both of said pair of devices.

2. In a device of the character described, the combination which includes a pair of discharge devices having cathode, anode and grid electrodes,

a 7 a first source of alternating potentials connected between the anode and cathode electrodes oi said devices, input terminals adapted to receive an alternating current control voltage of reversible phase, an amplifier having a pair of grid electrodes and a pair of anode electrodes, said grid electrodes being coupled to said input terminals, means coupling said first source of alternating potentials to said amplifier anode electrodes in opposite phase, means for applying the same rectified alternating and amplified control voltages developed by said amplifier between the oath e and grid electrodes 01' both said devices, whe y one or the other of said devices becomes nductive as a function oi the phase of the voltage applied to said input terminals, and output means in circuit with said devices operable in a first direction when one 01' said devices is conductive and in an opposite direction when the other 0! said devices is conductive.

3.In a device of the character described, the combination which includes a pair of discharge devices having cathode, anode and grid electrodes, a first source of alternating potentials connected between the anode and cathode electrodes of said devices and between the grid and cathode electrodes of said devices in phase opposition thereto,

'input terminals adapted to receive an alternating current control voltage of reversible phase, an amplifier having a pair 01' grid electrodes and a pair 01' anode electrodes, said grid electrodes being coupled to said input terminals, means coupling said first source 01' alternating potentials to said amplifier anode electrodes in opposite phase, means for app ying the same rectified alternating and amplified control voltages developed by said amplifier between the cathode and grid electrodes of both said devices, whereby one or the other of said devices becomes conductive as a function 01' the phase oi. the voltage applied to said input terminals, and output means in circuit with said devices operable in a first direction when one of said devices is conductive and in an opposite direction when the other of said devices is conductive.

4. A device of the character described in claim 3 in which, in the absence of an input control voltage, the peak amplitude of the rectified alter- 8 nating voltage applied between the cathode and grid electrodes of said devices is less than the peak value 0! the voltage applied to said grid electrodes from said first source, whereby both oi said devices'remsin non-conductive until an input control voltage is applied.

5. A system including a pair of discharge tubes each including at least an anode, a cathode, and a control grid; an alternating current supply source for said tubes, a motor coupled to said anodes so that the fiow oi anode current through one oi said tubes energizes said motor for rotation in one direction, and the fiow or anode current through the other 01' said tubes energizes said motor for rotation in the other direction; a source oi. alternating control voltage synchronous with said supply current, means applying said control voltage to both of said control grids in such manner that the instantaneous polarity of said control voltage at one of said control grids is the same as that at the corresponding anode, and the instantaneous polarity of said control voltage at the other 01' said control grids is opposite to that at the corresponding anode; and means for applying alternating bias voltage to the control grids or said tubes in phase opposition to the respective anode voltages 01' said tubes, and

further means applying a positive direct current bias to the control grids of both said tubes, said direct current bias having such a value that both said tubes are conductive when said control voltage is zero.

LOY E. BARTON.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS 

